"Split His Sides With Laughing"
Context: On his journey to Lady Dulcinea with the message from Don Quixote (which in reality he had forgotten to pick up), Sancho comes upon the curate and the barber from his master's village, looking for him. When they learn of the knight's proposed penance, they devise a scheme to end it. The barber will be the gentleman usher of the curate, who, dressed as a lady in distress, comes begging help from the Knight. The innkeeper's wife supplies costumes. For disguise, the barber puts on an ox's tale as a long beard. Then, as Cervantes reports:
. . . Sancho came up with 'em just upon their demur, and was ready to split his sides with laughing at the sight of these strange masqueraders.